Conventional radiation (e.g., gamma-ray) detectors for wellbore formation measurement (“well logging”) typically include a packaged photomultiplier (“PMT”) and scintillation crystal. The most common scintillation crystal is thallium-activated sodium iodide, NaI(Tl), which is a hygroscopic material and must be protected from moisture. Consequently, such NaI(Tl) crystals are typically packaged in a hermetically sealed container having an optical window to allow light to escape. The crystal is optically coupled to the interior surface of an optically transparent window in the container, typically with a clear silicone elastomer. This packaging method increases the number of optical interfaces, which causes a loss of light and detector resolution. Some concepts to improve the optical efficiency of the foregoing crystal packaging including, e.g., the development claimed in U.S. Pat. No. 7,321,123 (Simonetti et al.) incorporated herein by reference and also owned by the assignee of the present invention. In the approach of this patent, the scintillator crystal replaces an optical faceplate in the container and the photocathode of the PMT is directly deposited on the scintillator.
To construct a gamma-ray detector, in the typical detector, the exterior surface of the crystal container window is coupled optically to an exterior window of the photomultiplier (PMT), again using a clear silicone elastomer. For light generated within the scintillation crystal to reach the photocathode of the photomultiplier (PMT), it must pass through five interfaces: two on the optical coupling between scintillator and the scintillation crystal container window, two on the optical coupling between crystal container window and the PMT window, and one between the PMT window and the photocathode of the PMT. At each interface, only a fraction of the light is transmitted, while reflected light may be eventually re-reflected back toward the interface or it may be absorbed within the various optical media and thereby lost. It is advantageous to reduce the number of optical interfaces between the scintillation crystal and the photocathode of the PMT as this will reduce the amount of reflected light and therefore increase the amount of light that reaches the photocathode. Increasing the amount of light reaching the photocathode will improve the gamma-ray resolution and increase the signal-to-noise ratio as long as other parameters, such as photocathode quantum efficiency, are equal.
St. Gobain, a supplier of scintillator crystals, has published brochures describing “integrated” detectors in which an entire photomultiplier and scintillator are packaged together in a common hermetically sealed housing. Similar “integrated detectors” are also sold by GE-Reuter Stokes. Such a system also has only three optical interfaces as described above. However, the foregoing identified systems each has deficiencies with respect to shock-induced noise. This type of noise typically is produced by flexing of the optical coupling or scintillation crystal with the resultant emission of light as a result of the mechanical stress applied to the scintillator crystal. In the St. Gobain scintillation detector structure, the mass of the scintillation crystal and the mass of the PMT are disposed on either side of an optical coupling, and a shock, whether axial or lateral, will generate slight movement of the crystal and the PMT with respect to each other, and emit shock-induced light in the process. An additional disadvantage of the foregoing structure, in particular for LWD/MWD applications, is that the PMT needs to be surrounded by shock absorbing materials inside the housing. Outgassing of the shock absorbing material can damage the reflector, the optical coupling and can lead to an early failure of the PMT. For some very reactive scintillator material reactions with outgassing products may tarnish the scintillator surface and degrade the scintillator performance.